Why don't the current robot arms cover all ranges of workspace?

Recently my director professor Ilian Bonev opened a discussion in LinkedIn following our discussion in the CoRO lab. I couldn’t find the discussion so I decided to reopen a new one. The question was: Why aren’t there smaller-than-small industrial robot arms?

“Smaller than small” is a philosophical question. Let’s revise the question as an engineering one:

“Why don't the current robot arms cover all ranges of workspace with acceptable accuracy? “

For instance, the question is how much accuracy is feasible/acceptable for an arm with 1m maximum reachable radius (MRR)? To answer this question we need to define a parameter such as MRR per accuracy.

From manufacturing point of view, there are 2 essential restrictions: Mechanical restrictions and actuators restrictions.

Mechanical restrictions includes the accuracy of the gears, bearing etc. in related range. It is always difficult to have accurate gears and bearing in small ranges. One problem arises from high effect of backlash in different sizes of gears. The backlash-free gears have more complexity in their mechanical structures which reduce the life-time and increases the complexity of the system as well as the price. Recently HD released small Harmonics Drives to the market. They are also going to release the micro Harmonic Drives. So it seems that we will see smaller arms in near future. However, the flexibility in Harmonic Drives is still an issue. It seems that the nonlinear flexibility and hysteresis effect of Harmonic Drivers in smaller sizes is a challenge in accurate position controlling.

It is also difficult to find the accurate actuators in smaller sizes. It is almost impossible to have accurate controllable AC servo motor in centimeter dimensions. The DC stepper motors have less accuracy and high power failure in smaller size. We have smaller sizes of DC motors including brushed and brushless DC motor, with acceptable performance. Recently, improvement in controls of brushless DC motors make it possible to use them in small and medium sizes of robotics arms. In micro and nanometer range it is almost impossible to have good quality electromagnetic motors. The alternative option could be to use the piezoelectric actuators. They have low speed, zero backlash and high torque as well as high accuracy in the ranges of nanometers. Therefore, there is no need to use power transmission systems and velocity reducer. It is feasible to have piezoelectric actuators in nanometer ranges. Therefore, it seems that the only choice for the micro and nanometer range of manipulators is piezoelectric actuators. Piezoelectric actuators have some disadvantages such as short life time resulting from friction depreciation, difficulty of implementation of drivers and controller due to oscillation in natural frequency of actuator, non-linearity, and last but not least high prices.

Yousef B. Bedoustani

https://sites.google.com/site/cablerobots/